manuals: Fix syntax of list width

Signed-off-by: Graham Percival <gperciva@tarsnap.com>
Reviewed by: mhorne
MFC after: 3 days
Sponsored by: Tarsnap Backup Inc.
Pull Request: https://github.com/freebsd/freebsd-src/pull/1455

(cherry picked from commit 9fd66e40cac02691d36386cf275d23367e34fc43)